Analog phase shifters or modulators are well known, as disclosed for example in U.S. Pat. Nos. 4,288,763; 4,638,629; 4,837,532; 5,014,023 and 5,453,720. Such phase shifters utilize hyperabrupt varactor diodes known in the art and similar to that set forth in a paper by Niehenke et al., entitled Linear Analog Hyperabrupt Varactor Diode Phase Shifters, 1985 IEEE MTT-S Digest, pp 657-660. Further, abrupt and hyperabrupt varactors having predetermined capacitance-voltage characteristics as a function of doping profile are taught in U.S. Pat. No. 3,914,708, entitled, "BI-STATE Varactor Phase Modulation Network and Process for Constructing Same, issued to Stover, et al., U.S. Pat. No. 5,014,018, entitled Nonlinear Transmission Line for Generation of Picosecond Electrical Transients, issued to Rodwell, et al., and U.S. Pat. No. 5,557,140, entitled Process Tolerant, High Voltage, Bi-Level Capacitance Varactor Diode, issued to Nguyen. These aforesaid patents and publications being incorporated herein by reference thereto.
Still another example is U.S. Pat. No. 5,119,050, entitled Low-Loss 360 Degree X-Band Analog Phase Shifter, issued to Upshar, et al. This latter mentioned patent employs a pair of variable reactance networks directly connected to a 3-dB quadrature signal coupler or power divider to produce a 180 degree variable phase shift, and a second pair is directly connected to another 3-dB quadrature signal coupler to obtain an additional 180 degree phase shift. Impedance matching networks are also employed to reduce reflections and insertion loss.
In the just aforementioned patent, in order to obtain a phase shift range of greater than 180 degrees, two 3-dB couplers are required along with two additional variable reactance networks. These additional components lead to high insertion losses, reduction in linearity, and reduced bandwidth. Furthermore, the complexity caused by the increased component count diminishes the viability of a low cost and reliable production of a Monolithic Microwave Integrated Circuit (MMIC) implementation.
In one embodiment of the present invention, the variable reactance devices are varactor diodes which are selected so as to have a predetermined capacitance voltage characteristic so as to offset other circuit component induced non-linearities so as to enhance the linearity performance of the phase modulator. In accordance with one aspect of the present invention the varactor diode devices may be constructed so as to have a selected doping profile so as to yield the desired capacitance-voltage characteristic.